In principle, a seismic survey represents an analysis of the earth's geologic structure as indicated by seismic reflections from impedance discontinuities at lithologic interfaces. The analysis is influenced by seismic wave propagation velocities respective to the successively deeper geologic formations. A precisely-timed seismic source event, such as the ignition of buried explosives in a shallow borehole or a controlled mechanically-induced continuous vibration is launched at a precisely known location and time. The seismic source unit together with its controller, the seismic event generator, are designated collectively as the seismic source.
Seismic wave reflections from this man-made seismic event are detected by a multiplicity of geophone or hydrophone sensor arrays located in a more-or-less orderly grid over the area of interest. A series of such seismic source events is initiated at varying positions over the area of interest. The positions of the seismic sensor arrays may be shifted to better receive the seismic reflections of interest prior to each successive seismic source event. The location of each seismic sensor array and each source event is precisely mapped.
As a seismic wave from the timed event travels out from the source, reflections from that original seismic wave return to the surface where they are detected by the seismic sensor arrays. The sensor arrays respond to the receipt of a wave with a corresponding analog electrical signal. These analog signals are received by data acquisition modules that digitize and record the analog signal stream for retransmission to a central recording unit. Together, the seismic sensor array and the data acquisition module to which it connects may be termed the seismic receiver. Among the significant data digitized by data acquisition modules is the amplitude or the strength of the reflected wave. The time lapse between the moment the event occurred and the moment the amplitude of the wave is received is determined by the data acquisition system and is recorded either in explicit or implicit form. For each seismic source event and each array, amplitudes are sampled over a time range typically from zero to five seconds, for an impulsive source such as the buried explosive; or zero to twenty seconds for the continuous vibratory source, for example. Samples are typically repeated every 2 milliseconds, thus generating from two to ten thousand samples per seismic source event per source array in representative cases for impulsive and vibratory sources.
In a single survey, there may be thousands of seismic source events each with thousands of seismic sensor arrays. Consequently, the data flow must be orderly and organized. For example, the data acquisition modules transmit digital sensor signal values in digital data packages containing a predetermined number of digital data bits. Each of these data packages may carry the identity of the specific seismic sensor array from which the data originates and the time it was received by the array in addition to the seismic signal amplitude value. The acquisition modules are programmed to transmit data packets respective to each seismic sensor channel at a predetermined frequency. The variable data in a data packet represents an instantaneous snapshot of the analog signal flow from the array channel. There may be numerous individual seismic sensor arrays transmitting respective analog signals to the data acquisition module on the same communication channel.
Managing an orderly flow of this massive quantity of data to a central recording unit requires a plurality of geographically-distributed digital signal processing devices. The data acquisition modules convert the array analog data to digital data and transmit the digital data packets along receiver line cables (wired) or radio transmission (wireless) channels. Cables may be of various designs including both electrical conductor and fiber optic. Wireless channels are typically conventional radio but could also include light wave transmission.
There may be numerous data acquisition modules transmitting data packets along a single receiver line or channel. Typically, two or more receiver lines connect with line tap units that further coordinate the data packet flow of numerous additional line tap units along a base transmission line for receipt by a central recording unit. The base line may have a higher speed transmission capability than the receiver line to facilitate the flow requirement.
One of the key difficulties of a widely distributed seismic data acquisition system is that the transducers which measure the seismic vibrations of the earth must be very accurately timed, relative to a system-wide master clock. Furthermore, the devices that initiate the seismic signals must likewise be very accurately timed according to the same master clock reference. Generally acceptable timing accuracy is on the order of 50-100 microseconds, although accuracy as low as 1 millisecond can be tolerated in some circumstances. Less accurate timing can result in signal degradation in the various stages of processing to which the measured seismic reflections are subsequently subjected. The ultimate goal, to image the subsurface geologic layers, may be severely compromised by errors in timing accuracy of the recorded data.
A seismic data acquisition system may have many thousands of arrays of transducers (termed seismic sensor arrays) in contact with the earth with all of them being simultaneously measured and recorded. Many thousands of such recordings, each with a different seismic source location, are made during the course of a single 3D seismic survey. All of the recorded data may be combined in the imaging process. Difficulties in guaranteeing accurate timing arise due to the wide geographic dispersal of the seismic sensor arrays, often over varying and difficult terrain. The seismic sources are also positioned widely and initiated once for each recording, thus many thousands of times during the course of a typical survey project. The distances and obstacles separating the seismic receivers and sources make the synchronization of these seismic survey system elements very challenging.
It would thus be desirable to have a synchronization method and suitable equipment and software that could be used under a very wide range of conditions, including both wired and wirelessly connected network elements, to provide highly accurate and reliable synchronization of both seismic receivers and seismic sources in a networked total system. Such a system has been invented and is described in the remainder of this document.